The invention relates to a method for molding plastic, in which method plastic material is supplied to an injection space with injection means comprising at least one conical stator and at least one conical rotor, the rotor being rotated in such a way that as the rotor rotates, it supplies plastic material to be molded, so that the plastic material passes through the injection space and is compressed into a shape determined by a mold.
The invention also relates to an apparatus for molding plastic, the apparatus comprising an injection space, at least one conical stator and at least one conical rotatable rotor, the stator and the rotor forming means for supplying plastic material via the injection space, and means for compressing the plastic material into a shape determined by a mold.
The invention further relates to a plastic product produced with the method according to the invention.
DE 3,814,571 discloses a piston engine injection press. This injection press comprises a plasticizing screw in which the material to be molded is plasticized and with which it is supplied to a container via a valve. The container comprises an associated piston which can be moved with an actuator, so that when the container contains a sufficient amount of the material to be molded, the material is molded by means of the piston. The apparatus is cumbersome due to its deconcentrated structure. It is also difficult to adjust the apparatus, which is therefore not suitable for producing complicated technical products. The mass to be injected is also rather non-homogenous.
WO 85/03027 discloses an injection press wherein a screw is placed inside a cylindrical body. The material to be melted is supplied with a feed worm to a feed hopper and with the feed hopper to the space between the screw and the cylindrical body. The screw can be rotated, whereupon the screw thread transports the material to be melted forward and extracts new raw material from the hopper. Further, heating resistors are positioned around the body, so that when the screw transports the material forward, the raw material melts by the action of the heating resistors and the heat resulting from friction. At the same time as the screw rotates it also moves backwards, whereupon molten mass fills the space in front of the screw head. When a sufficient amount of material has been transported to the front of the screw, the rotation of the screw is terminated and the screw is pushed forward, whereupon the molten raw material flows via an injection aperture into the mold. This equipment is rather large and therefore expensive due to the length of the screw. Further, the batches of material injected at a time are rather small. Also, the orientation in the material of the product to be injected cannot be determined in any way.
WO 93/19923 discloses an injection press comprising a cylindrical body with a cylindrical screw movable axially and rotatably in it. The material to be supplied is fed with a feed hopper into the exterior of the screw. The screw, and partly the body, comprises threads with which the material to be supplied can be moved towards the front of the injection press. The interior of the screw comprises a core enabling the adjustment of the volume of the container to which the material to be injected is conducted. When the container contains a sufficient amount of the material to be injected, the material is pushed through a nozzle into the mold by means of the screw or a combination of the screw and the core. The apparatus is complicated and therefore difficult and expensive to manufacture. Further, the length of the screw and the core increases the manufacturing costs of the apparatus. Also, the orientation of the material of the product to be molded cannot be controlled in any way with this apparatus.
U.S. Pat. No. 4,519,976 and EP 0,619,172 disclose an injection press comprising a conical rotor outside which there is a conical stator, so that when the rotor rotates, it supplies material to be molded to the injection space. The conical structure does provide the injection press with a short and simple construction, but in the injection-molding apparatus according to the aforementioned references it is very difficult to arrange the heating and/or cooling functions of the material to take place from the inside of the cone formed by the supply means, i.e. through the rotating rotor. Also, it is difficult to provide high molding pressures in the feed opening between the rotor and the stator described in the references. Further, it is not possible to manufacture oriented products with the structure according to the EP reference, and the arrangement according to the U.S. reference requires the use of complicated and cumbersome mold tools.
The purpose of the present invention is to provide a method and an apparatus that do not comprise the aforementioned drawbacks. A further purpose of the invention is to provide a fiber-oriented plastic product produced with a mold.
The method according to the invention is characterized in that the rotor is placed outside at least one stator.
Further, the apparatus according to the invention is characterized in that at least one rotor is placed on the outside of at least one stator.
Further, the plastic product prepared with the method according to the invention is characterized in that the material of the plastic product is substantially oriented.
The essential idea of the invention is that the means for supplying material to the injection space include a conical rotor and a conical stator that are placed in such a way that the rotatable rotor is positioned outside the stator. The idea of an embodiment is that there is also another stator provided outside the rotatable rotor, so that the material to be molded flows both inside and outside the rotor. The idea of a preferred embodiment is that a piston with which the material supplied to the injection space is ejected into the mold is positioned in the centre of the apparatus. The idea of another preferred embodiment is that a mandrel is placed inside the piston, whereupon the material to be fed from between the rotor and the stators is supplied to a substantially tubular space between the mandrel and the frame, and outside the mandrel there is a cylindrical piston with which the material to be pressed is molded. The idea of a third preferred embodiment is that in the middle of the means for supplying the material there are blow means for compressing the material by means of compressed air into the shape determined by the mold.
The invention has the advantage that, when the material is supplied with conical stators and a rotor, the injection press can be provided with a short and simple structure. When the rotor is placed outside the stator, the means for controlling the temperature of the material to be molded can be placed easily in the middle of the injection press to warm the material to be molded from the inner stator. Also, it is easy to place the supply means for supplying the material to be molded in the inner stator, so that the injection press will have a simple and compact structure. When another stator is provided outside the rotor, the material to be molded can be made to flow both inside and outside the rotor, so that the rotor in a way floats inside the mass to be molded, and high molding pressures can be generated advantageously. Further, by positioning the mandrel inside the piston the orientation of the material to be fed, oriented by means of the stators and the rotors, can be maintained when the material is inserted into the substantially tubular space between the mandrel and the frame. Further, the orientation can be maintained by molding the product with a substantially tubular piston, thus producing fiber-oriented injection-molded products.